CLTw204

The car has a 3 year old AGM H7 battery. It will charge to 100% on the trickle charger, but was drained to 43% after 25 minutes of driving. Car has 75k miles and is a 2010 C250 (essentially a c300 of same year)

• No battery light in dash
• Battery reads 12.2+ volts when fully charged with trickle charger
• Battery reads 14.1 volts when engine is running (which indicates alternator is ostensibly working).
• No major drop (i.e., 2+ volts) when engine running, and turning on heat or headlights

What is the average lifespan on the alternators? Any ideas if the alternator is bad given the above set of facts? AutoZone meter gave multiple reads but seemed to land on bad alternator most often. The manager mentioned that the meter test for (3) parameters, so just because a battery read 14.1+ volts when running does not mean it is charging the battery.

TimC300

How are you reading the voltage, using a multimeter on the battery posts?

How do you know the battery is drained to 43% after 25 minutes of driving?

If the alternator is putting out 14 volts I don't see how its bad.

CaliBenzDriver

The battery charge level is a thorny topic with engine running and even more so while parked.

The alternator itself should be okay passed 100kMi but Valeo ALT output voltage is remotely managed by the ECU.


Normal running voltage range:
within 14.9V down to 12.6V

Drain while running:
less than 12.3V... 12.1V.... 11.7V

High AMPS are indication of battery charge status.

​​​​​​Observe the live voltage to confirm range.

JettaRed

iTrader: (1)

What trickle charger are you using?

CLTw204

@TimC300 Using a Multi-meter @ the posts to get my readings. And an AutoZone meter read the 43% battery charge @ the post.

@CaliBenzDriver - are you saying alternators on these cars are ticking time bombs past 60k miles?

@JettaRed a 1.5A Schumacher unit

Update: Alternator was tested today (@ the positive terminal on the alternator itself) and the system was determined to be functioning well. I will replace the battery and report back here after some days/weeks of driving to see if the new battery rids of the issue.

CaliBenzDriver

The VALEO alternator can have a normal lifespan 125 or 150 or 85kMi.

Lifespan depends on useage and service of serpentine tensioner, how often you WOT the engine?

The No1 trouble ALT seams to suffers is the voltage yoyo discharge below 12V but in reality that's caused by the Bosch ECU itself.

Your AGM is only 3 years old, it is a bit young unless it got drained too deep, too many times. Then a new Batt will restore more reliable starts.
🤞

JettaRed

iTrader: (1)

Yeah, not really robust enough for an AGM. Get the CTEK MXS 5.0 with 4.3 Amps. (CTEK is the manufacturer of the Mercedes branded chargers.)

TimC300

Im not a believer in battery testers that show depth of charge and state of charge, but who knows maybe autozone has a good one.

Is this whole issue just because of that one time you went to autozone to have the battery tested? Why did you have the battery tested in the first place?

When I thought I was having battery issues i bought the Ancel BM200 that you connect to the battery posts and it uses bluetooth to view everything on an app on your phone. That thing was supposed to show state of charge but it realized it was not accurate. The only thing it was good for is reading battery voltage. But it is very handy at that, I could see what the voltage was right before starting the car, then when I started the car the voltage would jump up telling me the alternator was working. can see results in real time, drive around and see exactly what the voltage it doing. I dont think it was accurate at showing battery health or state of charge because it was a generic tester, there were no settings to change it to agm battery. This tester was showing the battery charge was going down just parked.

To double check the results I bought another battery tester, the Konnwei KW208. That tester at least had settings to switch from lead acid to agm. This tester I would say was more in the ballpark of what the battery was doing.

I wont bore anyone more, you can read through that threat I made on the testers here: https://mbworld.org/forums/c-class-w...-readings.html

Scroll to post# 28 to get to the testers.


Here's a photo showing the different state of charge (SOC) results I was getting.



I do think the Ancel is great to connect and drive around to see what the voltage is doing, can see if the alternator is charging.

CLTw204

@TimC300 the car would not start randomly, hence why I had it tested.

Issue resolved with replacement of AGM battery. They are NOT all made the same. This was was from Pep Boys and was terrible. 3 years!

MODs,CLOSE the thread.

Moto_Guzzi

Good move replacing the battery for a steady voltage.